This invention relates to metallized electrical capacitor and more particularly to a partially impregnated metallized capacitor suitable for application voltages of above about 250 volts AC.
An important failure mechanism in capacitors is failure due to the deleterious effects associated with corona discharge and subsequent arcing. Corona discharge usually occurs in voids and air spaces at capacitor voltages above about 400 volts AC and following Paschen's law*. However, AC capacitors are usually designed so that they have a corona level well above the rated voltage because of transitory overvoltages or surges. For example, a 250 volt AC capacitor usually has a corona start voltage level of above about 400 volts. In order to minimize corona, higher voltage capacitors are usually fully and completely impregnated with a dielectric liquid impregnant which occupies most, if not all, voids and spaces which are corona sites and thus raises the corona level. FNT *On Sparking Over in Air, Hydrogen and Carbon Dioxide Under the Potentials Corresponding to Various Pressures (in German), Friedrich Paschen, Wiedemann Annalan der Physik und Chemie, Vol. 37, (1889) 69-96.
For operation at voltages below about 250 volts AC, a dry or unimpregnated metallized capacitor may be employed. A metallized capacitor is one wherein the electrode is usually a metal such as aluminum which is vacuum deposited as a thin film on a solid dielectric. This kind of capacitor has the advantage of self healing characteristics, when electrical breakdown or failure occurs, because the thin electrode metal supporting the arc vaporizes to thereby extinguish the arc or remove the fault from the system. Metallized capacitors are additionally advantageous because at the lower voltages they use thinner and non critical dielectrics operating at lower stresses.
Dry metallized capacitors for voltages in the range of 250 to 750 volts AC, and particularly from about 350 to 750 volts AC, have not found wide acceptance because with thin dielectric films under high stress and self clearing feature is too severe and there is excessive electrode erosion with loss of capacitance. There is also excessive gas generation at these voltages, and the temperature rise in these capacitors is too great. Dielectric liquid impregnation of these high stress capacitors is undesirable not only because of the increased economic factor but also because the impregnants, in the self clearing action, generate excessive gas and erode the metallized film. Consequently, one finds dry metallized capacitors only at lower voltages and thicker dielectric systems, i.e., low voltage stress systems, and liquid impregnated capacitors at higher voltages. However, dry metallized capacitors with highly stressed dielectric systems are more economical and otherwise advantages for medium voltage, 250 to 750 volt, AC systems.